September 30: Karan Jani (Vanderbilt University)
Title: GW190521 and Multi-Band, Multi-Messenger Astronomy
The new gravitational-wave signal GW190521 in LIGO and Virgo marks the first observational
detection of the elusive intermediate-mass black holes. In this talk, I would review the
implications of this historic discovery to cosmology and pair-instability supernovae theory.
Further, I would highlight how this discovery opens a unique science case for a joint multi-band,
multi-messenger gravitational-wave astronomy with the upcoming ESA/NASA space mission LISA and a
potential gravitational wave experiment on the Moon.
October 7: Wenbin Lu (Caltech)
Title: A Unified Picture of Fast Radio Bursts
Fast radio bursts (FRBs) are short duration (~ms), very bright radio transients. Their detection a decade ago was a major unexpected discovery in astronomy in decades. Hunting for FRBs and measuring their physical properties have become one of the leading scientific goals in astronomy. This effort has led to a rapidly growing sample with extremely diverse properties in luminosity (10^38 to 10^45 erg/s), duration (0.1 ms to 10 ms), and repetition rate (some objects have multiple bursts in an hour and many just one burst in a few years). I will present a study of their cosmological volumetric rate density and provide evidence that these bursts all belong to the same class of transients --- most likely all are repeaters. According to my model, disturbances close to the surface of a magnetar launch Alfven waves into the magnetosphere, which propagate to a distance of a few tens of neutron star radii and then produce coherent radio emission. The coincident hard X-rays associated with the Galactic FRB 200428 can be understood in this scenario. This model provides a unified picture for weak Galactic FRBs as well as the bright bursts seen at cosmological distances. If time allows, the polarization properties of FRBs will also be addressed.
October 14: Maya Fishbach (University of Chicago -> Northwestern)
Title: Astrophysical Lessons from Gravitational-Wave Populations of Black Holes and Neutron Stars
LIGO/Virgo detected eleven systems of merging black holes and neutron stars in their first two observing runs,
with several additional events published from the third observing run so far. Studying the masses, spins and
redshifts of these sources reveal several exciting features, including gap(s) in the mass spectrum, a preference
for equal-mass pairings, and signs that the merger rate evolves with redshift. I will discuss these features, and
how recent "exceptional" events from LIGO/Virgo fit into the population. These population properties may
reveal how black holes are made, as well as providing insight into nuclear physics and cosmology.
October 21: Kayhan Gultekin (University of Michigan)
Title: Supermassive Black Hole Pairs
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on the black hole outflows. Then I will concentrate on my group's recent work searching for dual and binary supermassive black holes along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binaries.
October 28: Michela Mapelli (University of Padova)
Title: The multifaceted formation of binary black holes
The latest results from the LIGO-Virgo collaboration challenge the astrophysical scenarios of black hole formation. In this talk, I will discuss the main astrophysical formation channels of binary black holes. On the one hand, models of stellar evolution and pair instability supernovae suggest a gap in the mass spectrum of black holes between ~60 and ~120 Msun. The boundaries of this gap depend on stellar rotation and on the efficiency of envelope removal. On the other hand, extreme dynamical processes in dense star clusters can fill the mass gap, via multiple stellar collisions and dynamical exchanges. Moreover, stellar dynamics enhances the formation of black hole -- neutron star systems with extreme (<1:10) mass ratios. Based on a data-driven model, I will discuss the merger history of dynamical versus isolated binary compact objects across cosmic time, and its dependence on the cosmic star formation rate and on the stellar metallicity.
November 11: Mohammad Safarzadeh (UC Santa Cruz)
Title: The Astrophysical Context of Gravitational Wave Events
We live in an era of breakthrough discoveries in gravitational waves (GW) astronomy.
Every month or so, such discoveries by LIGO/Virgo have been making headlines because these events' nature has been far from the expectations.
But why are we puzzled? And what is the road ahead for us to a deeper understanding?
I discuss two of the puzzling events that LIGO has discovered: 1) The most massive binary black hole merger with masses above the pair-instability limit. After an overview of the physics of pair-instability supernova, I will discuss how a broader perspective on the host environment of binary black holes can hold the key to understanding the nature of such massive systems. 2) The most massive binary neutron star merger. I will present possible scenarios to explain why we have not detected such systems in the radio observations before and how the key to understanding these events might lie in the r-process enrichment in the early universe and magnetic field evolution of neutron stars. Through these two examples, I will conclude that the synergy of electromagnetic-wave astronomy with gravitational wave astronomy is essential for gaining insights into the surprises that are being uncovered with these new observations.
November 18: Katelin Schutz (MIT)
Title: Making dark matter out of light: the cosmology of sub-MeV freeze-in
Dark matter could be a "thermal-ish" relic of freeze-in, where the dark matter is produced by extremely feeble interactions with Standard Model particles dominantly at low temperatures. In this talk, I will discuss how sub-MeV dark matter can be made through freeze-in, accounting for a dominant channel where the dark matter gets produced by the decay of plasmons (photons that have an in-medium mass in the primordial plasma of our Universe). I will also explain how the resulting non-thermal dark matter velocity distribution can impact cosmological observables.
November 25: [No seminar; Thanksgiving week]
December 9: Gongjie Li (Georgia Tech)
Title: Dynamical Origin and Habitability of Planets -- On the Spin-Axis Variations of Stars and Planets
The spin-axis dynamics of stars reveal histories on the formation mechanism of planetary systems. For instance, the observed spin-orbit misalignments suggest different pathways on the migration of planets. On the other hand, the variation of a planet's obliquity plays an important role in determining its climate, and the existence of massive satellites can influence such variations. For instance, the Earth's obliquity is stabilized by the Moon, and would undergo chaotic variations without the Moon. In this talk, I will discuss the origin of the ultra short period planets. Then, I'll present a simplified perturbative approach, and demonstrate that without the Moon, the stochastic change in the Earth's obliquity is sufficiently slow to not reach high values (>40 degrees) in billion-year timescales. In the end, I will discuss obliquity variations of exoplanets.